Method for creating branch seam with temporary plugging and pressure buildup using super absorbent resin

ABSTRACT

A method for creating a branch fracture with temporary plugging and pressure buildup using super absorbent resin, comprising: setting a fracturing packer to fracture a target layer by pad fluid to form a main fracture; injecting into the main fracture pad fluid mixed with salt-intolerant super absorbent resin and performing replacement till depth of the main fracture, and then injecting pad fluid mixed with strong salt-tolerant super absorbent resin; after the strong salt-tolerant super absorbent resin absorbs water and expands to form a temporary plugging layer, re-injecting the pad fluid to create a branch fracture with pressure buildup; and lifting injection pressure to break up the temporary plugging layer formed by the strong salt-tolerant super absorbent resin to form residue, transporting the residue to the depth of the main fracture to bridge the temporary plugging layer formed, and re-building up pressure to create a branch fracture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from Chinese Patent Application No. 201811495751.X, filed on Dec. 7, 2018. The content of the aforementioned application, including any intervening amendments thereto, is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The invention relates to the field of petroleum engineering hydraulic fracturing, and in particular, to a method for creating a branch fracture with temporary plugging and pressure buildup using super absorbent resin.

BACKGROUND OF THE INVENTION

With the acceleration of the exploration and development of unconventional oil and gas resources such as tight oil, shale gas and coalbed gas, the effective development of unconventional oil and gas resources has become a hot issue in the world. Hydraulic fracturing technology is an effective means for economic development of unconventional oil and gas. However, in order to form a sand packed fracture with high conductivity in a reservoir and greatly increase the yield of oil and gas wells, it is necessary to extrude a branch fracture in the stratum to form a complex fracture network system as much as possible. At present, the main disadvantage of conventional hydraulic fracturing technology is that it is often difficult to extrude a complex multi-level fracture network in reservoirs with abundant oil and gas and most fracture networks formed have a single shape and few horizontal and vertical expansions. Even some fracturing construction can only extrude one main fracture in the reservoir, which has almost no effect on the transformation of the reservoir, resulting in very low fracturing efficiency. This makes it difficult to transport and produce oil and gas, and the yield is not significantly improved after fracturing construction. How to extrude a branch fracture to form a multi-level fracture network system has become a bottleneck problem restricting the development of hydraulic fracturing technology. It is necessary to solve such problem for the development of unconventional oil and gas resources, solving China's energy security problems.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for creating a branch fracture with temporary plugging and pressure buildup using super absorbent resin. The method creates a branch fracture with temporary plugging and pressure buildup a plurality of times in a main fracture using the absorbance and expansion characteristics of super absorbent resin in conjunction with the characteristics of different salt-tolerant properties thereof, and can form a complex multi-level fracture network in one process, with little resin residue after fracturing and little damage to fracture conductivity, solving the problems faced by the above-mentioned conventional hydraulic fracturing technology, greatly increasing the effect of transformation of the reservoir and improving the rate of oil and gas recovery.

The invention provides a method for creating a branch fracture with temporary plugging and pressure buildup using super absorbent resin, including the following steps:

Step 1: setting a fracturing packer and using pad fluid to fracture a target layer to form a main fracture;

Step 2: injecting into the main fracture pad fluid mixed with salt-intolerant super absorbent resin and performing replacement till the depth of the main fracture, and then injecting pad fluid mixed with strong salt-tolerant super absorbent resin;

Step 3: after the strong salt-tolerant super absorbent resin absorbs water and expands and forms a temporary plugging layer at the front end of the main fracture, re-injecting the pad fluid to create a branch fracture with pressure buildup;

Step 4: lifting injection pressure to break up the temporary plugging layer formed by the strong salt-tolerant super absorbent resin, forming broken strong salt-tolerant super absorbent resin residue, transporting broken strong salt-tolerant super absorbent resin residue to the depth of the main fracture to be gradually bridged to the temporary plugging layer formed by the salt-intolerant super absorbent resin deep in the main fracture, and re-building up pressure to create a branch fracture;

Step 5: injecting pad fluid containing ammonium sulfate and hydrochloric acid to dissolve the salt-intolerant super absorbent resin and the broken strong salt-tolerant super absorbent resin residue, and increasing the length of the main fracture and of the branch fracture;

Step 6: injecting sand carrying liquid to fill the main fracture and the branch fracture, performing flow back of all the injected liquid, performing replacement and stopping the pump.

In the preferred method of the present invention, the salt-intolerant super absorbent resin in the second step may be selected from the following two types: 1) starch graft polyacrylamide with a water absorption ratio of 30 g/g-80 g/g, a particle size range of 0.5 mm-2 mm, and a compression strength of 0.05 MPa-1 MPa subject to saturated brine per cubic centimeter; and 2) cellulose grafted acrylamide with a water absorption ratio of 40 g/g-85 g/g, a particle size range of 0.5 mm-2 mm, and a compression strength of 0.04 MPa-1.2 MPa subject to saturated brine per cubic centimeter. The strong salt-tolerant super absorbent resin is a ternary copolymerized salt-tolerant acrylic acid super absorbent resin with a water absorption ratio of 150 g/g-270 g/g, a particle size range of 0.5 mm-1.5 mm, and a compression strength of 0.01 MPa-0.05 MPa subject to saturated brine per cubic centimeter. The brine in brine saturation for the starch grafted polyacrylamide, the cellulose grafted acrylamide, and the ternary copolymerized salt-tolerant acrylic acid super absorbent resin is standard brine of 0.9% NaCl.

In the preferred method of the present invention, the specific method of the second step is as follows: firstly, pad fluid of 5 m³-10 m³ mixed with the salt-intolerant super absorbent resin is continuously injected into the main fracture at a speed of 3 m³/min-10 m³/min, and then pad fluid of 5 m³-10 m³ containing no super absorbent resin is injected as replacement liquid to replace the salt-intolerant super absorbent resin to the depth of the main fracture, and finally pad fluid of 8 m³-10 m³ mixed with strong salt-tolerant super absorbent resin is continuously injected into the main fracture at a speed of 1 m³/min-3 m³/min, wherein a mass fraction of the salt-intolerant super absorbent resin accounting for pad fluid is 2%-8%, and a mass fraction of the strong salt-tolerant super absorbent resin accounting for pad fluid is 10%-15%.

In the preferred method of the present invention, the specific method of the third step is as follows: after the strong salt-tolerant super absorbent resin fully absorbs water and expands to form a temporary plugging layer, the pad fluid is injected into the main fracture at a speed of 5 m³/min-15 m³/min to build up pressure; during the pressure buildup process, it is found that bottom hole pressure continued to increase until breakthrough pressure began to decrease, indicating that temporary plugging pressure has reached formation fracture pressure and began to form a branch fracture, and then the pad fluid of 25 m³ is injected at a speed of 2 m³/min-5 m³/min to increase the length of the branch fracture.

In the preferred method of the present invention, if after the pad fluid is injected in the third step, bottom hole pressure cannot be lifted, indicating that the amount of the strong salt-tolerant super absorbent resin is insufficient to form the temporary plugging layer, and the solution is as follows: strong salt-tolerant super absorbent resin having a particle size of more than 1 mm and a mass fraction of 3%-5% is mixed into the pad fluid which is then continuously injected into the main fracture until the pressure starts to rise, and then the addition is stopped to build up pressure to create a branch fracture.

In the preferred method of the present invention, the specific method of the fourth step is: after the formation of the branch fracture at the front end of the main fracture is finished, the continuous injection of the pad fluid is started; pumping pressure is first increased gradually at a speed of 0.2 MPa/s; if it is found during the injection process that bottom hole pressure begins to greatly drop at a speed of not less than 0.5 MPa/s during continues increasing, indicating that the temporary plugging layer formed by the strong salt-tolerant super absorbent resin has broken and collapsed, pumping pressure is stopped to increase and the pad fluid is continuously injected at a speed of 2 m³/min-5 m³/min to transport the broken strong salt-tolerant super absorbent resin residue to the depth of the main fracture; if bottom hole pressure starts to increase again at a speed of not less than 5 MPa/min during transportation, indicating that the broken strong salt-tolerant super absorbent resin residue has been bridged to the temporary plugging layer formed by the salt-intolerant water absorbent resin, and temporary plugging is started to be formed, and the second pressure buildup and fracture branching is carried out in the same manner as in the third step.

In the preferred method of the present invention, in the fourth step, pumping pressure is gradually increased at a speed of 0.2 MPa/s, and if the pressure is still not lowered after the pressure is raised to 65 MPa, there are the following two solutions: 1) adding 0.05%-1% NaCl to the pad liquid, and lifting the osmotic pressure of the pad fluid so that the strong salt-tolerant super absorbent resin loses water and begins to shrink; and 2) adding 10%-15% ammonium persulfate to the pad fluid to dissolve the salt-tolerant super absorbent resin to help it break up and collapse.

In the preferred method of the present invention, in the fifth step, a mass fraction of the ammonium persulfate accounting for the pad fluid is 15%-25%, and a mass fraction of the hydrochloric acid accounting for the pad fluid is 5%-10%.

The technical principle of the invention is as follows: the super absorbent resin is a novel polymer material; the material has strong water absorption and the larger the water absorption amount, the larger the volume expansion ratio, and the compression strength is weakened after water absorption and saturation; and a new method for creating a branch fracture with multiple temporary plugging and pressure buildup in one-time construction of the present invention is designed utilizing these characteristics in conjunction with the characteristics of different salt-tolerant properties thereof. First, the salt-tolerant super absorbent resin has a slow water absorption rate in the pad fluid, and it is difficult to form a temporary plugging due to the small amount of water absorption. Therefore, the salt-intolerant super absorbent resin is first transported to the depth of the main fracture, and is slowly swelled after water absorption. Then, the strong salt-tolerant super absorbent resin which quickly absorbs water of a great volume in the pad fluid and can easily and quickly form temporary plugging is injected, and a temporary plugging layer is formed with rapid pressure buildup at the front end of the main fracture, and then pressure buildup and fracturing are performed. As the resin absorbs more water, the compression strength thereof decreases. After fracturing is completed, the pressure may be lifted to cause the temporary plugging layer thereof to collapse and break up to form residue. The residue is replaced by the pad fluid to the depth of the fracture, and the residue is gradually bridged to the salt-intolerant super absorbent resin which absorbs water and swells deep in the fracture. Thereafter, temporary plugging is formed again to perform pressure buildup and fracturing for the second time.

It can be seen from the above technical solution that the method for creating a branch fracture with temporary plugging and pressure buildup using super absorbent resin provided by the present invention at least has the following beneficial effects.

First, fracturing may be performed many times in one construction, which overcomes the technical drawbacks that the previous temporary plugging and fracturing method can only perform fracturing once after plugging the main fracture.

In the present invention, temporary plugging may be performed at the front end and at the depth of the main fracture, resulting in a larger fracture network area and stronger connection between reservoirs, which provides more channels for oil and gas transportation and greatly increases the production of oil and gas wells.

Second, the super absorbent resin absorbs water and does not absorb oil, and does not react with the reservoir during the whole construction process, without damaging the reservoir, and also does not need to be prepared during the entire construction process. The super absorbent resin can be used as needed. The construction process is simple. After the completion of the construction, the super absorbent resin is dissolved by a mixed solution of ammonium persulfate and hydrochloric acid, and almost no resin residue exists after flow back, with little damage to fracture conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the principle of creating a branch fracture for the first time of the present invention;

FIG. 2 is a schematic view showing the principle of creating a branch fracture for the second time of the present invention.

Reference signs in the figures: 1 main fracture, 2 strong salt-tolerant super absorbent resin, 3 salt-intolerant super absorbent resin, 4 strong salt-tolerant super absorbent resin broken residue, 5 branch fracture.

DETAILED DESCRIPTION OF THE INVENTION

In order to make the technical problems, technical solutions and beneficial effects of the present invention more clear, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Embodiment 1

A method for creating a branch fracture with temporary plugging and pressure buildup using super absorbent resin, comprises the following steps:

Step 1: setting a fracturing packer, injecting pad fluid of 45 m³, and using the pad fluid to fracture a target layer to form a main fracture 1;

Step 2: selecting starch graft polyacrylamide with a particle size of 0.7 mm, a water absorption ratio of 45 g/g, and a compression strength per cubic centimeter in saturated brine of 0.06 MPa-1 MPa as salt-intolerant super absorbent resin 3; selecting a ternary copolymerized salt-tolerant acrylic acid super absorbent resin with a particle size of 0.5 mm, a water absorption ratio of 175 g/g, and a compression strength per cubic centimeter in saturated brine of 0.02 MPa as strong salt-tolerant super absorbent resin 2; continuously injecting pad fluid of 7 m³ mixed with salt-intolerant super absorbent resin 3 into the main fracture 1 at a speed of 4.5 m³/min; then injecting pad fluid of 7 m³ without containing super absorbent resin to push salt-intolerant super absorbent resin 3 into the depth of the main fracture 1; finally continuously injecting pad fluid of 8 m³ mixed with strong salt-tolerant super absorbent resin 2 into the main fracture 1 at a speed of 1.5 m3/min; then closing the well for 2 minutes such that strong salt-tolerant super absorbent resin 2 absorbs water sufficiently and expands, wherein salt-intolerant super absorbent resin 3 accounts for 4% of the mass of the pad fluid, and strong salt-tolerant super absorbent resin 2 accounts for 10% of the mass of the pad fluid.

Step 3: after strong salt-tolerant super absorbent resin 2 absorbs water sufficiently and expands and forms a temporary plugging layer, injecting the pad fluid into the main fracture 1 at a speed of 6.5 m³/min to build up pressure, finding that bottom hole pressure continuously increases during the injection process and then begins to decrease after 5 min since the injection, which indicates that the temporary plugging pressure has reached formation fracture pressure and the formation of a branch fracture 5 begins, and thereafter injecting the pad fluid of 25 m³ at a speed of 2 m³/min to increase the length of the branch fracture 5;

Step 4: after the filling of the branch fracture 5 is completed, continuing to inject the pad fluid, gradually increasing pumping pressure at a speed of 0.2 MPa/s; if pumping pressure is still not lowered after being raised to 67 MPa during the injection process, beginning to add 0.05% NaCl into the pad liquid to lift the osmotic pressure of the pad fluid; after 7 min since the addition, finding that bottom hole pressure begins to greatly drop at a speed of 0.9 MPa/s during, indicating that the temporary plugging layer formed by the strong salt-tolerant super absorbent resin 2 has broken and collapsed, continuously injecting the pad fluid at a speed of 2 m³/min to transport the broken strong salt-tolerant super absorbent resin residue 4 to the depth of the main fracture 1; after 15 min, bottom hole pressure starting to continuously increase again at a speed of 7 MPa/min, indicating that the broken strong salt-tolerant super absorbent resin residue 4 has been bridged to the temporary plugging layer formed by the salt-intolerant water absorbent resin 3, and temporary plugging is started to be formed, then starting to inject the pad fluid into the main fracture 1 at a speed of 8.5 m³/min to build up pressure; finding that bottom hole pressure continuously increases during the injection process and then begins to decrease after 15 min since the injection, which indicates that the pressure has reached formation fracture pressure and the formation of a branch fracture 5 begins, and thereafter injecting the pad fluid of 25 m³ at a speed of 2 m³/min to increase the length of the branch fracture 5;

Step 5: injecting pad fluid containing ammonium sulfate (a mass fraction of 17%) and hydrochloric acid (a mass fraction of 7%) to dissolve the salt-intolerant super absorbent resin 3 and the broken strong salt-tolerant super absorbent resin residue 4, and increasing the length of the main fracture 1 and of the branch fracture 5;

Step 6: injecting sand carrying liquid to fill the main fracture 1 and the branch fracture 5, performing flow back of all the injected liquid, performing replacement and stopping the pump.

It has been found upon detection that the method for creating a branch fracture with temporary plugging using super absorbent resin has formed a new complex fracture network at the front and deep ends of the main fracture, greatly increasing the connectivity of the branch fracture and unused area of the reservoir and the natural fractures, and greatly improving the rate of oil and gas recovery.

Embodiment 2

A method for creating a branch fracture with temporary plugging and pressure buildup using super absorbent resin, comprises the following steps.

Step 1: setting a fracturing packer, injecting pad fluid of 37 m³, and using the pad fluid to fracture a target layer to form a main fracture 1;

Step 2: selecting cellulose grafted polyacrylamide with a particle size of 1.5 mm, a water absorption ratio of 75 g/g, and a compression strength per cubic centimeter in saturated brine of 1.2 MPa as salt-intolerant super absorbent resin 3; selecting a ternary copolymerized salt-tolerant acrylic acid super absorbent resin with a particle size of 1.2 mm, a water absorption ratio of 245 g/g, and a compression strength per cubic centimeter in saturated brine of 0.05 MPa as strong salt-tolerant super absorbent resin 2; first continuously injecting pad fluid of 6 m³ mixed with salt-intolerant super absorbent resin 3 into the main fracture 1 at a speed of 5.5 m³/min; then injecting pad fluid of 9 m³ without containing super absorbent resin to push salt-intolerant super absorbent resin 3 into the depth of the main fracture 1; finally continuously injecting pad fluid of 7 m³ mixed with strong salt-tolerant super absorbent resin 2 into the main fracture 1 at a speed of 2.5 m³/min; then turning off the bump for 1.5 minutes such that strong salt-tolerant super absorbent resin 2 absorbs water sufficiently and expands, wherein salt-intolerant super absorbent resin 3 accounts for 7% of the mass of the pad fluid, and strong salt-tolerant super absorbent resin 2 accounts for 15% of the mass of the pad fluid.

Step 3: after strong salt-tolerant super absorbent resin 2 absorbs water sufficiently and expands and forms a temporary plugging layer, beginning to inject the pad fluid into the main fracture 1 at a speed of 5.5 m³/min to build up pressure, finding that bottom hole pressure continuously increases during the injection process and then begins to decrease after 5 min since the injection, which indicates that the pressure has reached formation fracture pressure and the formation of a branch fracture 5 begins, and thereafter injecting the pad fluid of 25 m³ at a speed of 3 m³/min to increase the length of the branch fracture 5, and then injecting sand carrying fluid to fill the fracture;

Step 4: after the filling of the branch fracture 5 is completed, beginning to continuously inject the pad fluid, gradually increasing pumping pressure at a speed of 0.2 MPa/s; after bottom hole pressure quickly rises to 57 MPa during the injection process, finding that bottom hole pressure begins to greatly drop at a speed of 0.5 MPa/s, indicating that the temporary plugging layer formed by the strong salt-tolerant super absorbent resin 2 has broken and collapsed, then continuously injecting the pad fluid at a speed of 3 m³/min to transport the broken strong salt-tolerant super absorbent resin residue 4 to the depth of the main fracture 1; after 12 min, bottom hole pressure starting to continuously increase again at a speed of 8.2 MPa/min, indicating that the broken strong salt-tolerant super absorbent resin residue 4 has been bridged to the temporary plugging layer formed by the salt-intolerant water absorbent resin 3, and temporary plugging is started to be formed, then starting to inject the pad fluid into the main fracture 1 at a speed of 7.5 m³/min to build up pressure; finding that bottom hole pressure continuously increases during the injection process and then begins to decrease after 14 min since the injection, which indicates that the pressure has reached formation fracture pressure and the formation of a branch fracture 5 begins, and thereafter injecting the pad fluid of 25 m³ at a speed of 3 m³/min to increase the length of the branch fracture 5; and then injecting sand carrying fluid to fill the fracture;

Step 5: injecting pad fluid containing ammonium sulfate (a mass fraction of 21%) and hydrochloric acid (a mass fraction of 4%) to dissolve the salt-intolerant super absorbent resin 3 and the broken strong salt-tolerant super absorbent resin residue 4, and increasing the length of the main fracture 1 and of the branch fracture 5;

Step 6: injecting sand carrying liquid to fill the main fracture 1 and the branch fracture 5, performing flow back of all the injected liquid, performing replacement and stopping the pump.

It has been found upon detection that the method for creating a branch fracture with temporary plugging using super absorbent resin has formed a new complex fracture network at the front and deep ends of the main fracture, greatly increasing the connectivity of the branch fracture and unused area of the reservoir and the natural fractures, and greatly improving the rate of oil and gas recovery.

As described in the above embodiments, the method for creating a branch fracture with temporary plugging using super absorbent resin may effectively create a complex multi-level fracture network, increasing the conductivity between the branch fracture and unused area of the reservoir, and greatly improving the rate of oil and gas recovery.

The above concerns only the preferred embodiments of the present invention, and is not intended to limit the present invention. Therefore, any modification, equivalent substitution and improvement made using the contents of the description and drawings of the present invention should be included in the scope of protection of the present invention. 

1. A method for creating a branch fracture by temporary plugging and pressure buildup using absorbent resin, comprising the following steps: Step 1: setting a fracturing packer and using pad fluid to fracture a target layer to form a main fracture; Step 2: injecting into the main fracture pad fluid mixed with a first absorbent resin and adding a pad fluid free of absorbent resin to displace the pad fluid mixed with the first absorbent resin until the pad fluid mixed with the first absorbent resin reaches a position which is 67%-90% of the depth of the main fracture, and then injecting pad fluid mixed with a second absorbent resin; Step 3: after the second absorbent resin absorbs water and expands and forms a temporary plugging layer at a position which is 50-80 m away from an opening of the main fracture, injecting the pad fluid free of absorbent resin to create a branch fracture by building up the pressure; Step 4: lifting injection pressure of the pad fluid to break up the temporary plugging layer formed by the second absorbent resin, forming broken second absorbent resin residue, transporting the broken second absorbent resin residue to the the temporary plugging layer formed by the first absorbent resin to be bridged to the temporary plugging layer formed by the first absorbent resin in the main fracture, and re-building up pressure to create a branch fracture; Step 5: injecting pad fluid containing ammonium sulfate and hydrochloric acid to dissolve the first absorbent resin and the broken second absorbent resin residue; wherein the pad fluid containing ammonium sulfate and hydrochloric acid facilitates an increase in the length of the main fracture and of the branch fracture; and Step 6: injecting sand carrying liquid to fill the main fracture and the branch fracture, subjecting all the injected liquid, adding the pad fluid free of absorbent resin for displacement and stopping the pump; wherein in the fifth step, the ammonium persulfate is 15%-25% by weight of the pad fluid, and the hydrochloric acid is 5%-10% by weight of the pad fluid.
 2. The method according to claim 1, wherein the first absorbent resin in the second step is starch grafted polyacrylamide or cellulose grafted polyacrylamide, wherein the starch grafted polyacrylamide has a water absorption ratio of 1:30-80 by weight, a particle size range of 0.5 mm-2 mm, a compression strength of 0.05 MPa-1 MPa after per cubic centimeter of the starch grafted polyacrylamide is saturated with a brine, and the cellulose grafted polyacrylamide has a water absorption ratio of 1:40-85 by weight, a particle size range of 0.5 mm-2 mm, and a compression strength of 0.04 MPa-1.2 MPa after per cubic centimeter of the cellulose grafted polyacrylamide is saturated with the brine, and the second absorbent resin is a ternary copolymerized acrylic acid absorbent resin with a water absorption ratio of 1:150-270 by weight, a particle size range of 0.5 mm-1.5 mm, and a compression strength of 0.01 MPa-0.05 MPa after per cubic centimeter of the ternary copolymerized acrylic acid is saturated with the brine.
 3. The method according to claim 2, wherein the brine is normal saline of 0.9% NaCl.
 4. The method according to claim 1, wherein the the second step comprises the following steps: continuously injecting 5 m³-10 m³ of the pad fluid mixed with the first absorbent resin into the main fracture at a speed of 3 m³/min-10 m³/min, and then injecting 5 m³-10 m³ of the pad fluid containing no absorbent resin as displacement liquid to displace the first absorbent resin until the first absorbent resin reaches the position which is 67%-90% of the depth of the main fracture, and finally continuously injecting 8 m³-10 m³ of the pad fluid mixed with the second absorbent resin into the main fracture at a speed of 1 m³/min-3 m³/min, wherein the first absorbent resin is 2%-8% by weight of the pad fluid, and the second absorbent resin is 10%-15% by weight of the pad fluid.
 5. The method according to claim 1, wherein the the third step comprises the following steps: after the second absorbent resin fully absorbs water and expands to form a temporary plugging layer, beginning to inject pad fluid into the main fracture at a speed of 5 m³/min-15 m³/min to build up pressure; during the injection process, finding that bottom hole pressure continuously increased until breakthrough pressure began to decrease, indicating that temporary plugging pressure has reached formation fracture pressure and began to form a branch fracture, then injecting 25 m³ of the pad fluid at a speed of 2 m³/min-5 m³/min to increase the length of the branch fracture.
 6. The method according to claim 5, wherein if after pad fluid is injected in the third step, bottom hole pressure cannot be lifted, indicating that the amount of the second absorbent resin is insufficient to form a temporary plugging layer, mixing the second absorbent resin having a particle size of more than 1 mm and a mass fraction of 3%-5% into pad fluid which is then continuously injected into the main fracture until the pressure starts to continuously rise, stopping the addition, beginning to build up pressure to create a branch fracture.
 7. The method according to claim 1, wherein the the fourth step comprises the following steps: after the formation of the branch fracture at the front end of the main fracture is finished, beginning to continuously inject pad fluid; gradually increasing pumping pressure at a speed of 0.2 MPa/s; if it is found during the injection process that bottom hole pressure begins to greatly drop at a speed of not less than 0.5 MPa/s during increasing, indicating that the temporary plugging layer formed by the second absorbent resin has broken and collapsed, then stopping increasing pumping pressure and continuously injecting pad fluid at a speed of 2 m³/min-5 m³/min to transport the second absorbent resin residue to the depth of the main fracture; if bottom hole pressure starts to increase again at a speed of not less than 5 MPa/min during transportation, indicating that the broken second absorbent resin residue has been bridged to the temporary plugging layer formed by the first absorbent resin, and beginning to form temporary plugging, and performing pressure buildup to create a branch fracture for the second time in the same manner as in the third step.
 8. The method according to claim 7, wherein in the fourth step, pumping pressure is gradually increased at a speed of 0.2 MPa/s, and if bottom hole pressure is still not lowered after rising to 65 MPa, adding 0.05%-1% NaCl into pad liquid, and lifting osmotic pressure of pad fluid so that the second absorbent resin loses water and begins to shrink or adding 10%-15% ammonium persulfate into pad fluid to dissolve the second absorbent resin to help it break up and collapse.
 9. (canceled) 